Nuclear magnetic resonance imaging process and device

ABSTRACT

A method and an arrangement for imaging by means of nuclear magnetic resonance. A polarizing, homogenous, static magentic field is caused to act on the body (1) to be imaged, and then this magnetic field is reduced or switched off. Subsequently, a further magnetic field directed transversely to the polarizing magnetic field is caused to act on the body (1), and the magnetic fields forming by means of nuclear magnetic resonance are sensed by means of a SQUID gradiometer (5), and the image is formed from the signals of the gradiometer.

The invention relates to a method for imaging a region of a body bysensing magnetic fields forming on account of nuclear magneticresonance, by means of a gradiometer and processing the signals obtainedwith this gradiometer in a data processing device to give an image, thebody being subjected to a homogenous magnetic field, and a furthermagnetic field in which spatial gradients of the field intensity arepresent being caused to act on the body.

Methods of the afore-mentioned type are known in which the body of whichan image is to be made by means of nuclear magnetic resonance isconstantly subjected to a homogenous static magnetic field, and in whicha further magnetic field comprising spatial gradients is allowed to acton the body, and also a high frequency electromagnetic field is allowedto act on the body, the amplitude of the high frequency field in therespective body region being maintained as constant as possible, and thefrequency of this field being slightly varied corresponding to thegradient of the further magnetic field according to the resonanceconditions of the atoms present at the various sites of the body. Withthis known method, various problems occur on account of the action ofthe high frequency field, among which the often undesired warming onaccount of the high frequency field and the reduction of the fieldamplitude of the high frequency field in the interior of the bodyresulting from the skin effect are particularly to be mentioned.

It is an object of the invention to provide a different concept methodof the initially defined type, in which the detrimental problems of theknown methods are largely eliminated.

The inventive method of the initially defined kind is characterized inthat at first the body to be examined is subjected to a homogenouspolarizing static magnetic field and subsequently the field intensity ofthis magnetic field is reduced, preferably is brought to zero, and afurther static magnetic field whose field lines extend transversely tothe field lines of the said polarizing magnetic field and in which agradient is present is caused to act on the body, and that the componentof the magnetic fields thereby forming in the body on account of nuclearmagnetic resonance and extending in the direction of the field lines ofthe polarizing magnetic field is sensed by a highly sensitivegradiometer whose detection coils do not have any signal emittinginductive coupling with the polarizing magnetic field. With this method,the afore-mentioned object can well be met. No high frequency field isrequired, and it is possible to work with comparatively low fieldintensities of the polarizing magnetic field, while nevertheless afavorable signal-noise-ratio can be obtained. The nuclear spin of theatoms of the body portion to be imaged is polarized by the polarizingmagnetic field, and then this magnetic field is reduced or switched off,and the further magnetic field which is directed transversely to thepolarizing magnetic field is caused to act, resulting in a precessionmovement of the nuclear spin in the further magnetic field, theprecessing movement in turn forming a magnetic field which is sensed bythe gradiometer. If the said further magnetic field is orientedperpendicularly to the polarizing magnetic field, there results from theprecession movement an amplitude maximum of the magnetic field or signaldetectable by means of the gradiometer in the direction of thepolarizing magnetic field (z-direction). The signals emitted from thegradiometer are processed into an image by means of a data processingdevice.

Preferably, a SQUID gradiometer is used as the gradiometer. This resultsin a high sensitivity, particularly in the low frequency range of directcurrent up to approximately 10 kHz which is important here.

The configuration of the gradiometer which may be of the first, second,or of a higher, in particular the third, order is selected such that thedetection coils of the gradiometer do not have a signal emittinginductive coupling with the polarizing magnetic field. Prferably it isprovided that a gradiometer is used whose detection coils do not haveany signal emitting inductive coupling with the further static magneticfield, either.

In this manner, an overload of the SQUID is reliably avoided, and thedead times are reduced, thus enabling more rapid working.

The good sensitivity of the SQUID at low frequencies can be aided byfiltering out the high frequency to eliminate disturbing high frequencyinfluences. By a respective configuration of the input coils of themagnetometer, care may be taken with a view to disturbing externalfields that only the low frequency portions of the fields acting on thegradiometer are sensed.

As regards the polarization time, preferably the homogenous polarizingmagnetic field is kept constant for a span of time that is approximatelytwice to three times that of the longitudinal relaxation time T₁, andsubsequently is reduced, preferably switched off.

The invention also relates to an arrangement for carrying out the methodaccording to the invention. This arrangement is characterized in thatthe arrangement comprises a polarization magnet forming a homogenousstatic magnetic field, which polarization magnet is connected to aswitching means for reducing or switching off the magnetic flow, and amagnetic device suitable for forming a further static magnetic fieldextending transversely to the field of polarization and includingspatial gradients, which further magnetic device likewise is connectedto a switching means, as well as a highly sensitive gradiometer designedto sense field changes in the direction of the field of polarization,and, furthermore, a data processing device for forming an image from thegradiometer signals.

Preferably, the gradiometer is a SQUID gradiometer. For decoupling it ishere advantageous if the gradiometer comprises an adjustablecompensation coil next to the input coils, a compensation currentdetermined by the current of the polarization magnet and/or by thecurrent of the magnetic device being feedable to said adjustablecompensation coil.

In the drawing, an exemplary embodiment of an arrangement for carryingout the method according to the invention is schematically illustrated.

In the drawing, the body to be examined is denoted by 1, thepolarization magnet by 2, the magnetic system for forming the furthermagnetic field by 3, a switching device for the polarization magnet 2and the magnet system 3 is denoted by 4, and a gradiometer by 5. Thegradiometer 5 is a SQUID gradiometer having a SQUID 6 and a non-magneticcryostat 7. A control device for the SQUID & is denoted by 8, and a dataprocessing device for forming an image from the gradiometer signals andfor controlling the magnetic fields is denoted by 9. A preferablyprovided adjustable compensation coil 10 is supplied with a compensationcurrent that is dependent on the currents of the magnetic devices 2 and3.

Arrow B_(p) shows the direction of the polarizing magnetic field, andarrow B_(r) indicates the direction of the above-mentioned furthermagnetic field.

We claim:
 1. A method of imaging a region of a body by sensing magneticfields generated by nuclear magnetic resonance in said body, whichcomprises the steps of(a) subjecting the body to a first homogenousstatic magnetic polarization field, and then reducing the fieldintensity of said first static magnetic polarization field, (b) causinga further static magnetic field to act on said body,(1) the field linesof the further static magnetic field extending transversely to the fieldlines of the first static magnetic polarization field, and (2) agradient of the field intensity being present therein, (c) sensing fromthe magnetic fields generated in the body by nuclear resonance acomponent extending in the direction of the field lines of the firststatic magnetic polarization field by a highly sensitive SQUIDgradiometer,(1) the SQUID gradiometer having detection coils that haveno signal-emitting inductive coupling with the first static magneticpolarization field, and (d) processing the signals generated by theSQUID gradiometer in a data processing device to give an image.
 2. Amethod according to claim 1, wherein the field intensity of said firstpolarizing static magnetic field is reduced to zero.
 3. A methodaccording to claim 1, wherein the polarizing magnetic polarization fieldhas a field intensity in the range of from 10 mT to 100 mT is used.
 4. Amethod according to claim 1, wherein the gradiometer is used whosedetection coils do not have any signal emitting inductive coupling withthe further static magnetic field, either.
 5. A method according toclaim 1, wherein the homogenous magnetic polarization field is keptconstant for a span of time that is approximately twice to three timesthat of the longitudinal relaxation time T₁, and subsequently isreduced.
 6. A method according to claim 1, characterized in that onlythe low frequency portions of the fields acting on the gradiometer aresensed.
 7. An arrangement for imaging a region of a body by sensingmagnetic fields generated by nuclear magnetic resonance in said body,which comprises(a) a polarization magnet forming a first homogeousstatic magnetic polarization field,(1) the polarization magnetic beingconnected to a switching means for reducing the magnetic flow of thepolarization magnet, (b) a magnet device for producing a further staticmagnetic field extending transversely to the polarization field andwhich includes spatial magnetic field gradients,(1) the magnetic devicebeing connected to a switching means, (c) a highly sensitive SQUIDgradiometer having detection coils arranged and positioned to sensechanges in the direction of the polarization field, and (d) a dataprocessing device for forming an image of a region of said body fromsignals of said SQUID gradiometer.
 8. An arrangement according to claim7, wherein the SQUID gradiometer comprises an adjustable compensationcoil next to input coils, a compensation current determined by thecurrent of the polarization magnet being feedable to the detectioncoils.
 9. An arrangement according to claim 7, wherein the SQUIDgradiometer comprises an adjustable compensation coil next to inputcoils, a compensation current determined by the current of the magneticdevice being feedable to the detection coils.
 10. An arrangementaccording to claim 7, wherein the SQUID gradiometer comprises anadjustable compensation coil next to input coils, a compensation currentdetermined by the current of the polarization magnet and of the magneticdevice being feedable to the detection coils.